This invention pertains generally to energy storage devices, particularly high specific power and high energy density electrochemical capacitors known as supercapacitors, and specifically to a method of making active materials or electrodes for the same. There is a need for a rechargeable energy source that can provide high power, can be recharged quickly, has a high cycle life and is environmetally benign for a myriad of applications including defense, consumer goods, and electric vehicles. Double layer capacitors are rechargeable charge storage devices that fufill this need.
A single-cell double layer capacitor consists of two electrodes which store charge (these are called the "active" materials), separated by a permeable membrane which permits ionic but not electronic conductivity. Each electrode is also in contact with a current collector which provides an electrical path to the external environment. The electrodes and the membrane are infused with an electrolyte, and the entire assembly is contained in inert packaging. Multiple cells may be connected in series or in parallel in the final packaged unit.
Applying an electric potential across the electrodes causes charge to build up in the double layer which exists at the electrode/electrolyte interface of each electrode. This process can continue until a condition of equilibrium has been reached, i.e., the current drops to zero. The capacitance, or amount of charge that a capacitor can store, is directly related to the surface area of the electrodes. Therefore, electrodes made from conductive materials that possess high surface area (&gt;100 m.sup.2 /g) are desirable. By employing various materials and fabrication means, capacitors have been developed which are capable of delivering very high specific power and energy densities.
Because carbon is chemically inert, has a high electronic conductivity, is environmentally benign and is relatively inexpensive, it is a desirable material for fabricating electrodes for supercapacitors. High surface area carbon powders are presently preferred for use in fabricating supercapacitor electrodes. The internal resistance of carbon powder electrodes is dependent upon the extent and quality of particle-to-particle contact. As the quality and extent of these contacts decreases, the internal resistance of the electrode increases, which in turn reduces the usable stored charge in the capacitor. In some applications the electrodes are maintained under high compression in an attempt to make them more conductive. Binders are often used to fabricate freestanding electrodes from carbon powders. However, the binders, generally being of a higher resistance than the carbon particles they surround, may increase the particle-to-particle resistance thereby degrading the performance of the electrodes.
In addition to methods well known in the art for fabricating high surface area carbon electrodes such as employing a binder, the use of carbon paste electrodes, or high pressure, other methods of fabricating these electrodes to improve their conductivity have been developed. U.S. Pat. Nos. 5,150,283 and 4,327,400 disclose electrodes composed of electrically conducting substrates into which or upon which carbon powder in various forms is impressed. A method of fabricating electrodes which have high specific surface area is disclosed in U.S. Pat. No. 4,597,028. Here activated carbon fibers are woven into a fabric which is used to fabricate electrodes. Compounds which improve the conductivity of carbon powder electrodes have been also employed as disclosed in U.S. Pat. No. 4,633,372. These methods suffer from the disadvantage that they require additional fabrication steps which may be expensive and complex. It has been recognized that one way to overcome the problems associated with carbon powder electrodes is to develop carbon in the form of a continuous, monolithic structure and prepared in such a way as to possess the desirable properties of high surface area and low electrical resistance. As illustrated in U.S. Pat. Nos. 5,260,855; 5,021,462; 5,208,003; 4,832,881; 4,806,290; and 4,775,655, carbon foams, aerogels and microcellular carbons have been developed which are useful as electrode materials in high energy capacitor applications, because they possess high surface area, low electrical resistance and adequate mechanical strength. While these materials represent an improvement over conventional carbon powder electrodes for supercapacitors, they have several disadvantages. Methods used to prepare carbon foams, aerogels and microcellular carbons require elaborate processing steps to prepare the precursor materials; among other things, the solvents must be completely removed from the precursor prior to the carbonization step. In order not to disrupt the microstructure of the polymer precursor, the solvent removal step must be done under carefully controlled conditions, using, for example, freeze drying or supercritical extraction. Furthermore, the solvents must either be disposed of or purified prior to reuse. In contrast to these earlier approaches, the method disclosed in the present invention requires no elaborate processing and no solvents.
Some of the earlier art requires a post-carbonization step known as activation to produce a carbon with a high surface area (&gt;100 m.sup.2 /g). The activation treatment generally involves exposing carbon to an oxidant which may be a gas or an oxidizing chemical. Activation is a disadvantage in that it constitutes an extra processing step, and in addition, it is a difficult process to do reproducibly--small amounts of impurities such as alkalai metals cause variations in the surface area obtained, and furthermore the activating agent can preferably react at the surface of a porous carbon, resulting in inhomogeneity. In contrast to this prior art, the method disclosed in the present invention produces a high surface area monolithic carbon in a process that requires no activation.